It is well known to those skilled in the art that offshore drilling platforms are provided with a main deck for supporting such functions and items as cranes, crew quarters, heli-pad, lifeboats, racks for horizontally storing drill pipes and riser pipes etc. Arranged above the main deck there is typically a drilling deck which supports a derrick. An example of a drilling platform with a drilling deck arranged above a main deck is shown in the patent application WO 00/49266 published on Aug. 24, 2000; see e.g. FIG. 1A and page 5 line 29 and forward.
An offshore drilling platform 100 with a drilling deck 140 arranged above a main 130 deck is also schematically illustrated by FIG. 1 herein. The side view of the offshore drilling platform 100 in FIG. 1 shows a pontoon 110a of a lower pontoon structure and two columns 120a, 120b that extend vertically upwards from the pontoon 110a to a main deck structure 130. The main deck 130 connects the upper portions of the columns 120a-120b to each other so as to form a globally strong and resilient platform design. The drilling deck 140 arranged above the main deck 130 is provided with a rotary table assembly 142 and a derrick 150. The main deck 130 is provided with an opening 132 (sometimes called a moon pool opening) that is vertically aligned with the rotary table assembly 142. In addition, a blow-out-preventer 156 (BOP) is stored on the main deck 130. As can be seen in FIG. 1, the vertical clearance between the main deck 130 and the drilling deck 140 is preferably large enough to accommodate a fully assembled BOP, which is skidded or otherwise moved to a position above the opening 132 in the main deck 130 before it is lowered to the sea bed through the opening 132. It should be observed that the BOP is typically 10-15 meters high with a weight of about 150-350 tons.
Designs as the one shown in FIG. 1 have several drawbacks, A first drawback is that the different levels between the drilling deck 140 and the main deck 130 involves an undesired lifting and climbing activity between the decks 130, 140 having a negative effect on the safety and the productivity. In addition, the design in FIG. 1 results in an undesired high centre of gravity for the drilling platform 100, which is particularly undesired in connection with floating offshore drilling platforms.
In this connection it should be mentioned that it is well known to those skilled in the art that offshore drilling platforms may be provided with an additional cellar floor arranged below the drilling deck. Such a cellar floor may e.g. be an additional floor arranged between the drilling deck and the main deck. An example of a cellar floor can be found in patent U.S. Pat. No. 3,981,369 (Bokenkamp). The drilling deck and the cellar floor are at different levels, which typically results in undesired lifting and climbing, or additional means for lifting and transportation to avoid manual lifting and climbing. Moreover, the vertical clearance between the drilling deck and the cellar floor is typically large enough to accommodate a fully assembled BOP, which results in a rather high design for the drilling platform giving it an undesired high centre of gravity.
Hence, there is a need for a design that provides a compact offshore drilling platform with a low centre of gravity and an improved working environment.